In the extensive prior art on Coriolis-type mass flow sensors with vibrating bodies in the form of by tubes, there are only two basic tube shapes, namely at least one straight tube or at least one arbitrarily bent or even twisted tube. Among the latter, the preferred shape is the U-shaped tube loop, i.e., one with a straight inlet-tube section having an inlet end, a straight outlet-tube section having an outlet end, and a tube bend connecting the inlet-tube section with the outlet-tube section.
With both tube shapes, the less the vibrating body is mechanically balanced, or in other words, the greater the mechanical nonequilibrium of the vibrating body, the more the measurement result will be dependent on the mounting position of the mass flow sensor. "Mechanically balanced", i.e., in mechanical equilibrium, means that the total momentum and the total angular momentum of the vibrating body must each be zero at all times.
The majority of currently commercially available Coriolis-type mass flow sensors implements this equilibrium by an axially symmetrical construction of the vibrating body; the latter has a plane of symmetry, and it is excited in a symmetrical vibration mode, so that it is balanced as perfectly as possible.
In most cases, this axial symmetry requires at least two parallel measuring tubes which vibrate in phase opposition in the manner of a tuning fork, so that the fluid stream has to be divided into at least two separate streams and recombined by means of suitable fittings. In some applications, such as in the sanitary area, this is considered to be disadvantageous and results in, among other things, a cost disadvantage.
On the other hand, the fittings can be avoided if the (two) measuring tubes are connected in series with respect to the fluid stream; this system is difficult to drain, let alone self-draining.
Among the measuring tubes which are not straight, the helical measuring tubes represent a shape described in several prior art references.
JP-A 61-290324 and the English-language abstract belonging thereto as well as U.S. Pat. No. 4,957,005 describe a mass flow sensor with a single helical measuring tube which, having a rigid bar interconnecting the individual loops at a point of their circumference, operates more or less like the measuring tubes of the above-mentioned axially symmetrical mass flow sensors having at least two tube loops vibrating in the manner of a tuning fork.
French Patent 1,139,048 describes (probably for the first time) a gyroscopic mass flow sensor
which is designed to be installed in a conduit of a a given diameter through which flows a fluid to be measured, PA1 with a single measuring tube having essentially the shape of a circular cylindrical constant-pitch helix with several turns, PA1 with means which excite the measuring tube into vibrations, and PA1 with means which determine the mass flow rate of the fluid. PA1 which is designed to be installed, by means of flanges, in a conduit of a given diameter through which flows a fluid to be measured, PA1 with a single measuring tube having essentially the shape of a circular cylindrical helix with approximately one and a half turns and fixed at its ends in the respective flanges, PA1 with an external support tube having its ends fixed to the measuring tube near to the respective flanges, and PA1 with means which excite the measuring tube into vibrations about an axis which is perpendicular to, and does not intersect, the helix axis. PA1 which is designed to be installed in a conduit of a given diameter through which flows a fluid to be measured, PA1 with a single measuring tube having essentially the shape of a circular cylindrical helix with one and a half turns and fixed at its respective ends, PA1 with an internal, straight support element having only its ends fixed to the respective ends of the measuring tube, and PA1 with means which excite the measuring tube into vibrations perpendicular to the centerline of the measuring tube and parallel to the longitudinal axis of the support element. PA1 which is designed to be installed, e.g., by means of flanges, in a conduit of a given diameter through which flows a fluid to be measured, PA1 with a single measuring tube fixed at its ends in the respective flanges, the vibrating portion of said measuring tube having the shape of a circular cylindrical constant-pitch helix with at least one turn, PA1 with an external support tube whose ends are fixed to the measuring tube near to the respective flanges, or to the respective flanges themselves, PA1 with an internal support element having only its ends fixed to the measuring tube near to the respective flanges, PA1 with connecting elements which are distributed along the vibrating portion of the measuring tube, interconnect said portion and the support element, and permit only all those modes of the vibrating portion of the measuring tube in which the centerline of said vibrating portion remains, as far as possible, on the rest-position cylindrical enveloping surface of said modes, and PA1 with means which excite the vibrating portion of the measuring tube into resonance vibrations perpendicular to its centerline.
EP-A 210 408 describes a mass flow sensor working on the Coriolis principle
This mass flow sensor is obviously not balanced; its measurement results, particularly their zero stability, are therefore highly dependent on its mounting position.
The same applies to the Coriolis-type mass flow sensor disclosed in U.S. Pat. No. 4,733,569
Despite the above-described disadvantages, a mass flow meter with a single helical measuring tube is attractive for several reasons, namely because it permits a greater length of the measuring tube for a given size of the meter and, thus, provides high sensitivity, because its measuring tube can further be made in a continuous bending/twisting process and, thus, with high precision (as is possible with straight and circular-arc-shaped measuring tubes), and because it is self-draining in at least one mounting position.
It is, therefore, an object of the invention to improve mass flow sensors with a helical measuring tube so that they have largely balanced modes of vibration regardless of the density of the fluid while retaining the self-draining capability in at least one mounting position.